1. Field of the Invention
The present invention relates to an image pickup apparatus which utilizes a charge coupled solid-state image pickup device of a frame interline transfer system (FIT-CCD), and particularly relates to an image pickup apparatus having an electronic shutter function to perform picking up of an image.
2. Description of Background Art
A charge coupled solid-state image pickup device for use in such an image pickup apparatus has a structure as shown in FIG. 5.
That is, a charge coupled solid-state image pickup device is constituted by a drain portion 1 for discharging unnecessary electric charges, a photo-detection portion 2 for photo-detecting an optical image of a subject, a storage portion 3 for temporarily holding signal charges for every picture element produced in the photo-detection portion 2, and a horizontal charge transfer line 4 for reading out the signal charges in the store portion 3. The charge coupled solid-state image pickup device is formed by a semiconductor production technique.
Further, describing the structures of the respective portions, first, in the photo-detection portion 2, a plurality of photo-diodes are arranged in a matrix in vertical and horizontal scanning directions V and H. For example, in the case of a primary color stripe filter, as shown in FIG. 3, color filters of red (R) are laminated on the respective surfaces of a group of photo-diodes arranged in the first column, color filters of green (G) are laminated on the respective surfaces of a group of photo-diodes arranged in the second column, and color filters of blue (B) are laminated on the respective surfaces of a group of photo-diodes arranged in the third column. These three columns are arranged repeatedly in the horizontal scanning direction H. The photo-diodes correspond to the respective picture elements. Assuming that these photo-diodes are arranged in M rows (M is an even number) in the vertical scanning direction V, a photo-diode group in an odd numbered row numbered from the side of the drain portion 1 is regarded as a first field, and a photo-diode group in an even numbered row is regarded as a second field.
Vertical charge transfer lines l.sub.1 to l.sub.N are formed adjacently to photo-diode groups in the respective columns. Four-phase driving signals .phi.I1, .phi.I2, .phi.I3 and .phi.I4 are applied to a transfer electrode group (not shown) laminated on the upper surfaces of the vertical charge transfer lines so as to produce transfer elements to transfer signal charges in the vertical scanning direction V. Further, photo-shield layers are formed on the upper surfaces of all the vertical charge transfer lines 1.sub.1 to 1.sub.N to prevent light from striking the surfaces.
Further, transfer gates (for example, shown as represented by TG in FIG. 5) are provided between respective photo-diodes and charge transfer elements in vertical charge transfer lines adjacent thereto, for conducting and therefore transferring signal charges produced in the respective photo-diodes to the charge transfer elements in the vertical charge transfer lines. Gate electrodes for driving the transfer gates are formed integrally with the transfer electrodes in the vertical charge transfer lines, and the transfer gates are set in a conductive state by setting driving signals to a high voltage at a predetermined timing.
The drain portion 1 is constituted by a predetermined impurity layer formed so as to connect with one end of all the vertical charge transfer lines 1.sub.1 to 1.sub.N, for transferring unnecessary charges transferred through the vertical charge transfer lines, to a semiconductor substratum.
The storage portion 3 is constituted by a charge transfer line group provided continuously with the other ends of all the vertical charge transfer lines 1.sub.1 to 1.sub.N. Four-phase driving signals .phi.S1, .phi.S2, .phi.S3 and .phi.S4 are applied to a transfer electrode group (not shown) laminated on the upper surfaces of the vertical charge transfer lines so as to function to transfer signal charges from the photo-detection portion 2 in the vertical scanning direction V and hold the signal charges in a predetermined charge transfer element group by stopping the driving signals temporarily. It is therefore possible to produce a transfer element group to temporarily hold signal charges produced in photo-diodes of M/2 lines (that is, one field). Further, photo-shield layers are formed on the upper surfaces of all the vertical charge transfer lines to prevent light from striking the upper surfaces.
The horizontal charge transfer line 4 is connected with the ends of all the charge transfer lines of the storage portion 3 so as to transfer signal charges in the horizontal scanning direction H synchronously with two-phase driving signals .phi.H1A and .phi.H2 applied to a transfer electrode group (not shown) formed on the upper surface of the horizontal charge transfer line 4. The signal charges transferred synchronously with the two-phase driving signals .phi.H1A and .phi.H2 are impedance-converted in a floating diffusion amplifier 5 synchronously with a reset signal .phi.Rs and an output gate signal .phi.H1B, and are supplied to an output terminal 6 as a color signal every picture element.
FIG. 6 shows image pickup timing when respective fields are read in an electronic still camera or the like having an electronic shutter function.
Assume that a shutter release button of an electronic still camera is pushed at a point of time t1 in FIG. 6. Then, synchronously with the pushing of the shutter release button, four-phase driving signals .phi.I1, .phi.I2, .phi.I3 and .phi.I4 are set at predetermined voltage levels respectively so as to make only transfer gates corresponding to a first field conductive, so that unnecessary charges in a photo-diode group corresponding to the first field are transferred to vertical charge transfer lines.
Next, at a point of time t2 after a predetermined period .tau.o has elapsed from the point of time t1, the four-phase driving signals .phi.I1, ,.phi.I2, .phi.I3 and .phi.I4 are set at predetermined voltage levels respectively so as to make only transfer gates corresponding to a second field conductive so that unnecessary charges in a photo-diode group corresponding to the second field are transferred to the vertical charge transfer lines.
After unnecessary residual charges in all the photo-diodes are transferred to the vertical charge transfer lines by those transfer operations at the points of time t1 and t2, in a predetermined period .tau.1 from a point of time t3 to a point of time t4, the vertical charge transfer lines of the photo-detection portion 2 and the charge transfer lines of the storage portion 3 transfer all the unnecessary charges to the side of the drain portion 1 synchronously with driving signals .phi.I1 through .phi.I4 and .phi.S1 through .phi.S4 to thereby discharge the unnecessary charges.
Next, at a point of time t5 immediately after an exposure period .tau.3 established in accordance with the shutter speed has elapsed, the same charge transfer processing as that at the point of time t1 is performed. That is, the exposure period of respective photo-diodes corresponding to a first field is from the point of time t1 till the point of time t5, the point of time t1 being that when the four-phase driving signals .phi.I1, .phi.I2, .phi.I3 and .phi.I4 are set at predetermined voltage levels respectively so as to make only transfer gates corresponding to a first field conductive so that signal charges in a photo-diode group corresponding to the first field are transferred to the vertical charge transfer lines to thereby discharge signal charges.
Next, in a predetermined period .tau.4 from a point of time t6 till a point o time t7, the vertical charge transfer lines l.sub.1 through l.sub.N of the photo-detection portion 2 and the charge transfer lines of the storage portion 3 perform a charge transfer operation at a high speed synchronously with the driving signals .phi.I1 through .phi.I4 and .phi.S1 through .phi.S4, so as to transfer signal charges corresponding to the first field onto the charge transfer lines of the storage portion 3.
Next, the same charge transfer processing as that at the point of time t2 is performed at a point of time t8 immediately after an exposure period .tau.5 (here, .tau.5=.tau.3) established in accordance with the shutter speed has elapsed from the point of time t2 when unnecessary charges in the photo-diode group corresponding to the second field are discharged. That is, the exposure period of respective photo-diodes corresponding to a second field is from the point of time t2 till the point of time t8, the point of time t2 being that when the four-phase driving signals .phi.I1, ,.phi.I2, .phi.I3 and .phi.I4 are set at predetermined voltage levels respectively so as to make only transfer gates corresponding to a second field conductive so that signal charges in a photo-diode group corresponding to the second field are transferred to the vertical charge transfer lines to thereby discharge signal charges.
Next, in a predetermined period .tau.6 from a point of time t9 till a point of time t10, the transfer operation of the vertical charge transfer lines 1.sub.1 through 1.sub.N in the photo-detection portion 2 is stopped so that the signal charges corresponding to the second field are held in those vertical charge transfer lines, and at the same time, the storage portion 3 and the horizontal charge transfer line 4 are made to perform a charge transfer operation at a predetermined timing so that only the signal charges corresponding to the first field are read out to an output terminal 6.
Next, in a predetermined period .tau.7 from a point of time t11 till a point of time t12, the vertical charge transfer lines of the photo-detection portion 2 and the charge transfer lines of the storage portion 3 perform a charge transfer operation at a high speed synchronously with the driving signals .phi.I1 through .phi.I4 and S1 through .phi.S4, so as to transfer signal charges corresponding to the second field onto the charge transfer lines of the storage portion 3.
Next, in a predetermined period .tau.8 from a point of time t13 till a point of time t14, the storage portion 3 and the horizontal charge transfer line 4 perform a charge transfer operation at a predetermined timing so that the signal charges corresponding to the second field are read out to the output terminal 6.
As has been described, by performing read-out scanning as shown in FIG. 6, it is possible to obtain an artificial frame electronic shutter function having an exposure period from the point of time of a reset operation (the point of time when unnecessary charges are discharged) till the point of time when read-out scanning for every field is started.
However, since signal charges corresponding to a second field are read out after signal charges corresponding to a first field are read, as shown in the period of .tau.6 in FIG. 6, the signal charges corresponding to the second field are held in vertical charge transfer lines temporarily, so that a smear is mixed in the signal charges corresponding to the second field in this holding period (if in accordance with a standard television system, 1/60 second, that is, 16.7 mS). Accordingly, if a frame picture is reproduced on the basis of these read signals, there has been a problem that field flicker is produced between reproduced pictures of the first and second fields due to the luminance difference of smear components thereof. Moreover, in such a case where a subject having a spot-shaped portion of high light intensity is to be photographed, there has been a problem wherein blooming is produced since a plurality of smears are produced which overflow into the vertical charge transfer lines when the signal charges corresponding to the second field are being held in the vertical charge transfer lines.
Further, with increased shutter speed the influence of smear mixing increases. FIG. 7 is a diagram of a characteristic curve showing a level ratio by percent (smear mixing ratio) between picture signals read-out from first and second fields for every color according to the shutter speed when one and the same subject is taken. As shown in FIG. 7, the light intensity to a photo-detection portion increases as the shutter speed becomes high so that generation of smear increases. If the ratio of a smear component of a second field to a first field reaches more than about 1 percent, flicker can be recognized by human eyes so that deterioration of picture quality results.
Conventionally, therefore, it has been considered to provide an image pickup apparatus in which smear is prevented from occurring by partial use of a mechanical shutter with an electronic shutter.
This apparatus includes a mechanical shutter provided corresponding with the photo-detection surface side of a solid-state image pickup device, similar to that shown in FIG. 5, so that an electronic shutter starts the opening of the shutter and a mechanical shutter closes the shutter. In this technique, there is provided a so-called VOD by which unnecessary charges in photo-diodes constituting picture elements can be discharged to a semiconductor substrate side. Described on the basis of a timing diagram shown in FIG. 8, first, a mechanical shutter is made to be in an opened state so as to radiate an optical image of a subject onto a photo-detection surface of a solid-state image pickup device, and all the unnecessary charges are discharged to the semiconductor substrate side through the VOD structure in a predetermined short period .tau.1 from a point of time t1 synchronized with the pushing operation of a shutter release button.
Next, the mechanical shutter is closed at a point of time t3 immediately after the lapse of an exposure period .tau.2 established in accordance with the shutter speed from a point of time t2 when discharge of unnecessary charges is completed. Therefore, photo-diodes of the first and second field have the same timing and the same exposure period .tau.2.
Next, the same read-out scanning as that after the point of time t5 in FIG. 6 is performed (in processing after the point of time t5 in FIG. 8, the same points of time as those in FIG. 6 are referenced correspondingly).
Therefore, in the read-out scanning after the shutter is closed, the solid image pickup device is photo-shielded perfectly by the mechanical shutter, so that smears are not mixed in at all, and the problems of field flicker and the like can be solved.
However, in such a conventional image pickup apparatus using a mechanical shutter together with an electronic shutter, there has been a problem that high shutter precision cannot be obtained unless the mechanical shutter can perform a closing operation at an extremely high speed. That is, when a mechanical shutter in an opened state is closed at a certain point of time, the shutter does not close immediately, but photo-shield is achieved by the gradual movement of a photo-shield plate of the shutter from one end side to the other end side. Therefore, as shutter speed is higher, greater photo-shield precision of the mechanical shutter is required.